The polyvinylidene fluoride is a thermoplastic fluorinated polymeric membrane material with excellent weather resistance and chemical stability, which will not be corroded by acid, alkali and other strong oxidants and halogen, and it has high strength and wear resistance, widely used in industry, municipal wastewater treatment, medicine, food, gas filtration, molecular transfer printing technology and other industrial fields, which has wide development prospects and market values.
The polyvinylidene fluoride is mainly used for microfiltration and nanofiltration in the field of membrane, and its preparation processes are mainly the non-solvent induced phase separation (NIPS) and the thermal induced phase separation (TIPS). At present, the TIPS method is used more widely in industry, and this method uses the casting solution to make double diffusion and mass transfer of solvent/non-solvent with the surrounding, to change the originally homogeneous and stable solution to an unstable status. Thus the phase transformation occurs and it is phase-separated and solidified as the membrane finally. The polyvinylidene fluoride membranes used in the market are mostly prepared by this method, but this method also has some shortcomings, mainly including that there are often finger-like pores in the membrane pore structure, which will reduce mechanical strength and service life of the membrane. And the thermal induced phase separation (TIPS) mainly makes the phase separation by cooling, to form the pore structure, where the prepared membrane has a high strength, a membrane pore structure of homogeneous network-pore structure and other advantages, but this method has a high requirement for equipment, large energy consumption and a more difficult preparation method.
American patent U.S. Pat. Nos. 5,022,990 and 6,299,773 mix polyvinylidene fluoride resin, organic liquid and inorganic granular material, and after melting in a high temperature, a hollow fiber membrane and the likes are formed by model molding, to form a three dimensional network structure. But the addition of the inorganic granular material will reduce the mechanical strength of the membrane and the inorganic granular materials are difficult to be extracted in later period. American patent U.S. Pat. No. 6,013,688 adopts the acetone as the solvent and prepares a polyvinylidene fluoride membrane with good penetrability and high porosity through the non-solvent induced phase separation. But this technology needs to dissolve the polyvinylidene fluoride near the boiling point of acetone and the acetone is a volatile toxic substances, resulting in a large difficulty in industrial actual operation. Chinese patent CN104607063A uses the strong hydrophilic property of polyvinyl alcohol to modify the polyvinylidene fluoride membrane, adopts the blend polyvinyl alcohol, crosslinks and stabilizes the polyvinyl alcohol in the membrane, to form a network structure with the main polyvinylidene fluoride and retain its permanent hydrophilic property.